Method of processing steel sheet or strip

ABSTRACT

A METHOD OF PRODUCING FULLY-HARD, STRESS-RELIEVED PLAIN CARBON STEEL STRIP SUITABLE FOR USE AS CAN END-STOCK WHEREIN A LOW CARBON STEEL IS HOT ROLLED TO HOT BAND GAUGE AT TEMPERATURES WITHIN THE AUSTENITIC RANGE AND THEREAFTER RAPIDLY COOLED TO A TEMPERATURE BELOW THE AUSTENTIC RANGE PRODUCTING SMALL DISPERSED CARBIDES. THE STEEL IS COLD REDUCED TO FINAL GAUGE AND CONTINUOUS ANNEALED AT A TEMPERATURE BELOW ABOUT 1050*F. TO RELIEVE ROLLING STRESSES BUT AVOID SUBSTANTIAL RECRYSTALLIZATION. ON THE OTHER HAND, A STEEL STRIP SUITABLE FOR USE AS CAN BODY STOCK, I.E. HAVING LESS STRENGTH BUT GREATER FORMABILITY IF PRODUCED IF THE STEEL COOLED SLOWLY AFTER HOT ROLLING TO CAUSE LARGE AGGLOMERATED CARBIDES, AND IT THE FINAL CONTINUOUS ANNEAL IS AT A TEMPERATIVE BELOW ABOUT 1025*F., AGAIN TO RELEIVE ROLLING STRESSES BUT AVOID SUBSTANTIAL RECRYSTALLIZATION.

United States Patent US. Cl. 148-12 Claims ABSTRACT OF THE DISCLOSURE A method of producing fully-hard, stress-relieved plain carbon steel strip suitable for use as can end-stock wherein a low carbon steel is hot rolled to hot band gauge at temperatures within the austenitic range and thereafter rapidly cooled to a temperature below the austenitic range producing small dispersed carbides. The steel is cold reduced to final gauge and continuous annealed at a temperature below about 1050 F. to relieve rolling stresses but avoid substantial recrystallization. On the other hand, a steel strip suitable for use as can body stock, i.e. having less strength but greater formability, is produced if the steel cooled slowly after hot rolling to cause large agglomerated carbides, and if the final continuous anneal is at a temperature below about 1025 F., again to relieve rolling stresses but avoid substantial recrystallization.

This is a division of application Ser. No. 567,357, filed July 25, 1966, now abandoned.

This invention relates to improved methods for processing steel sheet or strip. More particularly, the invention relates to improved methods of producing full-hard, stressrelieved, plain-carbon steel strip. In accordance with the invention, there are provided methods for producing fullhard, stress-relieved strip (1) with characteristics such as to render the steel suitable for use in applications requiring high strength and good formability and (2) with characteristics such as to provide a steel with greater ductility at elevated levels of tensile strength. Still more particularly, the invention relates to improved methods for processing plain-carbon steel to provide steel suitable for use both as can end-stock and can body-stock and in other applications commonly calling for tin plate and tin mill black plate.

Conventional manufacturing processes for can endstock, and in particular the end stock for cans used to contain beverages and 'which may be subjected to high internal pressure, have required the use of high-nitrogen plain-carbon steels. Such steels have been furnished as annealed and completely recrystallized intermediate gauge product. Such annealing is accompanied by a lowering of the hardness from over 80, as measured on the Rockwell 30-T scale, to a level of about 60 on the same scale. A final cold reduction of from 30 to 50% is conventionally performed to achieve the required level of mechanical strength, and following the final cold reduction the steel may be tin coated as desired.

While the foregoing conventional processing yields material which enjoys some degree of commercial acceptance, the processing is cumbersome and expensive and is subject to a number of operating problems which add to the manufacturing cost. The present invention provides two methods of processing plain-carbon steel to produce full-hard, stress-relieved steel strip. In accordance with one embodiment of the invention to produce steel suitable for certain can end-stock applications, a

3,59l,427 Patented July 6, 1971 plain-carbon steel containing from about 0.07 to 0.1% carbon and only residual nitrogen, is hot rolled to hot band gauge at a finishing temperature in the austenitic range. After hot rolling, the steel is rapidly cooled from the austenitic range to a temperature not in excess of 1200 F., preferably 1050" to 1100 F., to produce hot-rolled steel having relatively small well-dispersed carbides. After conventional descaling, the steel is cold reduced to final gauge and is then continuously annealed at a temperature not exceeding about 1050 F., preferably 975 to 1000 F., sutficient to relieve rolling stresses but to avoid substantial recrystallization. In accordance with the second embodiment of the invention to produce full-hard, stress-relieved, plain-carbon steel suitable for can body applications and can end applications requiring somewhat less mechanical strength but greater formability than the beverage can end, a plaincarbon steel containing up to about .08% carbon, preferably about 0.05 to 0.08% carbon, and which contains only residual nitrogen, is hot roll-ed to hot band gauge at a finishing temperature in the austenitic range and then cooled from the austenitic range at a rate slow enough to produce relatively large agglomerated carbides. This is accomplished by utilizing a minimum of cooling water and coiling at temperatures not less than 1200 F., preferably not less than 1250 F. After conventional descaling, the steel is cold reduced to final gauge and is finally continuously annealed at a temperature not exceeding 1025 F., preferably 975 to 1000 F., suificient to relieve rolling stresses but to avoid substantial recrystallization. In practicing each of the aforementioned embodiments, the steel may be temper rolled to improve surface condition and flatness, but such temper rolling must be performed prior to final continuous annealing.

The product produced in accordance with the inven tion has improved flatness characteristics and strength. By means of the controlled, limited heat treatment following cold reduction, sufiicient formability is restored to the steel to permit fabrication without serious impairment of the high level of tensile stength of the cold reduced material. Flatness of the product is enhanced by continuously annealing rather than box annealing, and contiuous annealing under tension is critical in producing a product of acceptable presently commercial flatness.

Successful achievement of the strength and formability requirements in accordance with the invention depends to a large degree on the carbon content and carbide dispersion in the plaincarbon steel. It has been found that for the practice of the first of the aforementioned embodiments the carbon content must be very critically maintained within the range of from about 0.07 to 0.1% and a microstructure consisting of relatively small well-dispersed carbides must be achieved. Carbon contents above about 0.10% generally result in an excessively brittle steel, and carbon contents below about 0.07% result in increasingly low tensile strength. Similarly, the carbon content of the plain-carbon steel used in practicing the second embodiment of the invention should be not more than about 0.08% carbon and the carbides must be relatively large and agglomerated to assure the improved ductility at elevated levels of tensile strength which characterize this embodiment.

Whereas the conventional steel used in beverage can end-stock applications contains a nitrogen addition, it has been found that this is not necessary and only the residual amounts of nitrogen remaining in the steel should be pres ent. Since both open hearth and basic oxygen steels may be employed, the limit of the nitrogen content which may be contained will vary to some extent. However, it is not necessary in practicing the invention to add nitrogen to achieve the desired strength level.

In accordance with the invention, plain-carbon steel is processed into hot rolled strip of hot band gauge at a finishing temperature in the austenitic range.

To perform the method according to the first embodiment of the invention, after hot rolling, the steel is rapidly cooled from the finishing temperature to the coiling temperature in such a manner that iron carbides formed as a result of decreasing solubility of carbon at the lower temperatures are essentially of small size, and well-dispersed throughout the hot rolled microstructure. This is accomplished by utilizing a substantial amount of cooling after final hot working by means of conventional sprays n the runout table of a hot strip mill. The cooling is provided in such a manner so that the strip temperatures do not exceed about 1200 F. as the strip enters the coiling equipment. After conventional descaling, the strip is then cold reduced to final gauge for the end stock application. After cold reduction, the steel is heat treated under tension to improve its flatness and to restore suflicient formability to permit fabrication of, for example, beverage can ends without serious impairment of the strength inherent as a result of the cold reduction process. This is accomplished in a continuous annealing facility operated so as to effect a heat treatment in which substantially no recrystallization is produced in the microstructure. With a plain-carbon steel having the aforementioned critically limited carbon content, carbide dispersion and prior treatment such as described above, it has been found that annealing temperatures not exceeding approximately 1050 F. will provide adequate relief of the rolling stresses with no readily apparent recrystallization. As a result, a product is produced in which both the microstructure and physical strength approach those of cold reduced steel as contrasted with those of annealed or recrystallized steel.

As an illustration of this embodiment of the invention, a slab of plain-carbon steel with a ladle analysis of 0.096% carbon was hot rolled to a finishing temperature of 1550 F. (which is in the autenitic range) and to a hot band gauge of 0.080-inch. After hot rolling, the steel was rapidly cooled to a coiling temperature of 1100 F. The coil was subsequently pickled and descaled and then cold reduced to 0.0099-inch. After cold reduction, the steel was cleaned and then, as in the preferred embodiment, temper rolled for surface and flatness improvement. As a final step, the strip was stress relief annealed under tension at 1000 F. to relieve rolling stresses without substantial recrystallization and to enhance the flatness of the product. The properties after stress relief annealing were as follows:

Rockwell Hardness 30-T scale: 81.5 Tensile strength (L) at one end of the coil: 115,000 p.s.i.

Tensile strength at the other end of the coil: 117,000 p.s.i.

In accordance with the second embodiment of the invention to produce a steel suitable for use in can body applications and can end applications requiring somewhat less mechanical strength but greater formability than the beverage can end, it is necessary to use a plain-carbon steel having not in excess of about 0.08% carbon. After hot rolling as described above to a hot band gauge at a finishing temperature in the austenitic range, the steel is cooled at a rate slow enough to produce relatively large agglomerated iron carbides. In practice, this is accomplished by utilizing a substantially retarded cooling rate after hot rolling, and coiling at temperatures not less than 1200 F. Subsequently, the steel is descaled by conventional continuous pickling methods and cold reduced to final gauge. As a last step (after temper rolling as in the perferred embodiment) the steel is continuously annealed under tension at a temperature not exceeding 1025 F. to relieve rolling stresses without substantial recrystallization and to enhance the flatness of the product. The exact amount of the annealing depends upon the design and speed of the continuous annealing facility and the details of the steel chemistry and prior processing so as to effect heat treatment without obvious recrystallization of the cold worked microstructure. The exact amount of tension, as in the first embodiment, depends on the design of the continuous annealing facility and the specific material being processed, but in all cases is moderate in the sense that no stretching of the steel occurs other than that very small amount occurring in connection with the correction of minor flatness irregularities. Of course, where flatness of the product is not a factor, the heat treatment may be performed without regard to tension of the strip. The resultant material possesses a somewhat lower tensile strength than the product produced by the first method, but has greater ductility. By virtue of its improved ductility and its still relatively high strength level, such material is of value for a number of applications.

As an illustration of the second embodiment of the invention, a steel slab with a ladle analysis of 0.07% carbon was hot rolled at a finishing temperature of 1615 F. to a hot band gauge of 0.080-inch. After hot rolling, only a small amount of cooling Water was used and the steel was coiled at a coiling temperature of 1285 F. to produce a microstructure with relatively large, agglomerated carbides. Subsequent to coiling, the steel was de scaled by conventional continuous pickling methods and then cold reduced to .0099-inch. Following cold reduction, the steel was cleaned and temper rolled to improve surface characteristics and flatness. It was then continuously annealed at 1010 F. to relieve rolling stresses without obvious recrystallization. Properties of the strip produced were as follows:

Tensile strength (L): 98,000 p.s.i. Elongation in Z-inches: 2.5%

Strip which has been substantially cold reduced is prone to display a non-flat condition due in some measure to high residual stresses generated in the steel by the cold reduction operation. Even the temper rolling operation by means of which the desired surface finish is normally provided can readily generate stresses which in material of elevated strength characteristics are suflicient to cause non-commercially acceptable flatness. In order to avoid such difliculties and to achieve maximum flatness it has been found that it is advantageous that no cold working or temper rolling for surface finish or for other reasons, such as for flatness, be done after the final stress relief heat treatment. Where such operations are necessary, they should be conducted, in accordance with the invention, prior to the stress relieving heat treatment rather than after it. By so doing, any stresses set up in the steel by such processing are relieved during the subsequent heat treatment and do not interfere with product flatness.

I claim:

1. A'method of producing a fully-hard stress-relieved plain-carbon sheet steel containing up to about 0.08% carbon and only residual nitrogen which is suitable for use as can body-stock, the steps comprising hot rolling a plain-carbon steel to hot band gauge at a finishing temperature in the autenitic range, cooling the hot rolled steel from said austenitic range to a temperature not less than 1200 F. at a slow rate sulficient to produce relatively large, agglomerated carbides, cold reducing said hot rolled steel to final gauge, and continuously annealing the cold reduced steel at a temperature not exceeding about 1025 F. sufiicient to relieve rolling stresses but to avoid substantial recrystallization.

2. A method according to claim 1 wherein said steel is temper rolled prior to continuously annealing.

3. A method according to claim 1 wherein after hot rolling said steel is cooled from the austenitic range to a temperature not less than 1250 F.

4. A method according to claim 1 wherein said steel is continuously annealed under tension to enhance flatness at a temperature in the range of about 975 to 1000" F.

5. A method according to claim 1 wherein said steel contains from about 0.05 to 0.08% carbon.

References Cited UNITED STATES PATENTS Wean 14812t Campbell et a1. 14812 Burns et a1. 148---12 George et a1. 14812 Epstein et al. 14812 White 14812 6 3,161,225 12/1964 Ward et al. -a 148-12 3,260,623 7/1966 Klein 148-12 3,264,144 8/1966 Frazier et a1. 148-12 5 OTHER REFERENCES The Making, Shaping, and Treating Steel, 7th edition, p. 594.

10 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner 

